can I set quota for some fields? like for my event registration form, I want to limit the number of seats for 8am start time as 800, then the particip

If there are no existing submissions in your from, I would suggest that you change the "Start Time" field to either of the following widgets Appointment Slots, Ticket Purchases, or Gift Registry.

However, if there are existing submissions, you can just hide the current "Start Time" field (How-to-Hide-Form-Fields) then, add a new "Start Time" field using any of the mentioned widgets above.

With those widgets, you can limit the number of times an option can be selected.

If you have other questions or concerns, please feel free to let us know.